Add the events subsystem

Implied by video and joystick

Property definitions

sdl2 $ SDLInitFlags :: events
	# Add the events subsystem
	#
	# Implied by `video` and `joystick`
	fun events: SDLInitFlags `{ return self | SDL_INIT_EVENTS; `}
lib/sdl2/sdl2_base.nit:125,2--128,62